The present invention is generally directed to inserts for horizontal penetrations extending through the outer wall to the inner wall of cryostats, and more particularly to inserts having a vertical service stack and employing liquid helium as a coolant material.
In the generation of images in medical magnetic resonance diagnostic systems, it is necessary to provide a temporarily stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures, provides an advantageous means to produce such a field. Accordingly, for such MR imaging devices, a cryostat is employed. A cryostat contains an innermost chamber in which liquid helium, for example, is used to cool the superconductive materials. The cryostat itself, typically comprises a generally toroidal structure with other nested toroidal structures inside the exterior vessel, to provide the desired vacuum conditions and thermal shielding. Since it is necessary to provide electrical energy to the main magnet coil and to various correction coils employed in MR imaging and to replenish coolant material, it is necessary that there be at least one penetration through the cryostat vessel walls.
Present MR magnets with horizontal tube penetrations require two separate plugs, one to power the magnet as described and claimed in U.S. Pat. No. 4,535,596 and another plug to be used when the magnet is operating in the persistent mode providing high thermal efficiency as described and claimed in U.S. Pat. No. 4,516,404. The procedure to remove and insert these plugs is complex, requires the training of operators and can inadvertently admit air inside the cryostat. The use of a helium liquifier, which captures helium boiloff and returns liquid helium to the cryostat, with these plugs could result in contamination of the liquifier because of the admission of air into the system.
A horizontal cryostat penetration has the advantages of simplifying manufacturing, being easier to support during shipping, providing more overhead room and allowing easy access for service. A single penetration insert is highly desirable for servicing the magnet for all the required operations including cooldown, excitation, persistent mode operation, and shimming.
It is an object of the present invention to provide a single horizontal penetration insert with a vertical service stack through which the magnet can be powered or operated in a persistent mode without admitting air inside the helium vessel.
It is a further object of the present invention to provide a single horizontal penetration insert to function as an interface between the cryostat and a helium liquifier.